1
ANNEX 15
NATIONAL ANNEX
TO STANDARD
SFS-EN 1994-1-2EUROCODE 4: DESIGN OF COMPOSITE STEEL AND CONCRETE STRUCTURES
Part 1-2: Generalrules. Structuralfiredesign
Preface
This National Annex is used together with Standard SFS-EN 1994-1-2.
This National Annex sets out:
a) National parameters for the paragraphs of Standard SFS-EN 1994-1-2 where national selection is permitted.
National selection is permitted in the following paragraphs of Standard SFS-EN 1994-1-2:
- 1.1(16)
- 2.1.3(2)
- 2.3(1)P
- 2.3(2)P
- 2.4.2(3)
- 3.3.2(9)
- 4.1(1)P
- 4.3.5.1(10)
b) Guidance on the use of the Informative Annexes A, B, C, D, E, F and G and also on replacing the Informative Annex H.
1.1 Scope
1.1(16)
When designing in accordance with Standard SFS-EN 1994-1-2, the highest strength class of concrete is C50/60.
2.1.3 Parametric fire exposure
2.1.3(2)
No values are given for the average temperature rise Δθ1 and for the maximum temperature rise Δθ2 during the cooling phase of fire.
Explanation:
The requirement for separating function EI is only based on a standard fire and on temperature limits set by it.
The fire safety requirement is also deemed to be satisfied if the building is designed and executed based on design fire scenarios which cover the situations likely to occur in the said building. Satisfaction of the requirement is attested case-by case taking into account theproperties and use of the building (The NationalBuilding Codeof Finland E1:1.3.2).
2.3 Design values of material properties
2.3(1)P
The values of partial safety factors for mechanical properties of steel and concrete areM,fi,a= 1.0; M,fi,s= 1.0; M,fi,c= 1.0; M,fi,v= 1.0.
2.3(2)P
The values of partial safety factors for thermal properties of steel and concrete in the equations (2.2a) and (2.2b)areM,fi= 1.0.
2.4.2 Member analysis
2.4.2(3)
Note1: Partial safety factors of loads in accordance with the National Annex to StandardSFS-EN 1990 are used in Finland. An equivalent, conforming to Fig. 2.1 in Standard SFS-EN 1994-1-2,is set out in the following Fig.2.1(FI) where the reduction factor fihas been calculated as a function of the load ratio Qk,1/Gkusing the values in the Finnish National Annex toStandard SFS-EN 1990.
Note 2: As a simplification, the recommended valuefi = 0.65 is used excluding load class E (storage and industrial premises) in accordance with Standard SFS-EN 1991-1-1 where the value fi = 0.7 is used.
Fig.2.1(FI) Variationof the reduction factorfias a function of the load ratioQk,1/Gkwhen partial safety factors in accordance with the National Annex to StandardSFS-EN 1990 are used.
3.3.2 Normal weight concrete
3.3.2(9)
An upper limit value in accordance with expression (3.6a) in Standard SFS-EN 1994-1-2 is used for the thermal conductivityc of normalweight concrete.
4.1 Introduction
4.1(1)P
Advanced calculation methods may be used in Finland. Their validity is verified in accordance with paragraph 4.4.4.
4.3.5.1 Structural behaviour
4.3.5.1(10)
The values 0.5 and0.7times the systemlengthLare used for the buckling lengths Lei andLet.
Annexes A, B, C, D, E, F, G and I
Informative Annexes A, B, C, D, E, F, G and I may be used.
Annex H
Simple calculation model for concrete filled hollow sections exposed to fire all around the column according to the standard temperature-time curve
Informative Annex H is not used in Finland.
Simple design methods may be used for fire design of concrete filled hollow sections, the principles of which are set out in paragraph 4.3.5.1 of Standard SFS-EN 1994-1-2. These methods concern columns exposed to standard fire exposure on each side in the same way.
(1) The simple methods described are only used when designing columns in braced frames. The model should include data on the temperatures i, on which the calculations of the compressive resistance of the cross-section and effective flexural stiffness of the column are based.
(2) With simple methods, the design value for compression resistance of an axially loaded column is calculated as a buckling load from the expression:
(4.12)
where
is a reduction factor conforming to the buckling curve c set out in paragraph 6.3.1 of Standard SFS-EN 1993-1-1 corresponding to the relative slenderness , unless demonstrated reliably that some other buckling curve is more correct,
Nfi,pl,Rdis the fire design value for cross-sectional compression resistance.
(3) Cross-section is divided into parts according to the materials where “a” is used for steel profile, “s” for reinforcement and “c” for concrete.
(4) Fire design value for cross-sectional compression resistance is:
(4.13)
where
Ai,is the area of each part of cross-section at the temperature.
(5) Effective flexural stiffness is calculated from:
(4.14)
where
Ii,is the second moment of area of material partirelated to the consideredaxis,
i,is the reduction factor which is calibrated to match the compression resistances achieved in loaded fire tests so that the result is on the safe side.
Ec,sec,is the characteristic value of secant modulus of concrete during fire which is calculated by dividingfc, by compressioncu,.
(6) Critical buckling load during fire in accordance with the elasticity theory is
where
is the buckling length during fire calculated in accordance with paragraph 4.3.5.1(10).
(7) Relative slenderness during fire is:
where
Nfi,pl,Ris the value of cross-sectional resistance Nfi,pl,Rdin accordance with paragraph (4) when the partial safety factorsM,fi,a, M,fi,s andM,fi,care = 1,0.
(8) The design value Nfi,Rd, of compression resistance in an eccentrically loaded column is
wherefcd = fck/cis the design strength of concrete at normal temperature andMEd,1 = the maximum first order design moment due to design load at normal temperature. Npl,Rd andMpl,Rdare the design resistances of the column cross-section in accordance with Standard SFS-EN 1994-1-1.
NOTE 1:
The values of reduction factors i,are characteristic to each method and depend on the temperatures of steel section, reinforcement and concrete used in the method and also on the curve included in the buckling analysis.
NOTE 2:
Reduction factors used in the method may be calibrated on the basis of compression resistances achieved in fire tests of loaded columns so that, when calculating the values Nfi,pl,R and(EI)fi,eff, the temperatures included in the design model and the material characteristics corresponding to these set out in paragraphs 3.2.1, 3.2.2 and 3.2.3 in Standard SFS-EN 1994-1-2,are used.